Initiation of in situ combustion in carbonaceous stratum



Nov. 28, 1961 R. v. GERNER 3,010,513

INITIATION OF IN SITU COMBUSTION IN CARBONACEQUS STRATUM 2 SheetsSheet 1 Filed June 12, 1958 1 ifPRODUCTION OVERBURDENI BUSHING MATERIAL j l TAR SAND TAR SAND F/G. FIG. 2

INVENTOR. R.V. GERNER BY mow/w ATTORNEYS Nov. 28, 1961 R. v. GERNER 3,010,513

INITIATION OF IN SITU COMBUSTION IN CARBONACEQUS STRATUM Filed June 12, 1958 2 Sheets-Sheet 2 PRODUCTION TAR SAND BUSHING MATERIAL INVENTOR. R.V.GERNER BY KZ A TTORNEYS United States Patent 3,010,513 INITIATION OF IN SITU COMBUSTION IN CARBONACEOUS STRATUM Robert V. Gerner, Bartlesville, Ok1a., assignor to Phillips Petroleum Company, a corporation of Delaware Filed June 12, 1958, Ser. No. 741,589 7 Claims. (Cl. 166-11) This invention relates to a process or method for initiating in situ combustion in a permeable carbonaceous stratum.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move through the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and, in the case of inverse drive, upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein, followed by injection of air through the ignition borehole in the stratum, constitutes a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the Stratum. In this type of operation the stratum frequently plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difficulty and to permit the continued progress of the combustion zone through the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed through the stratum to the combustion zone from one or more surrounding boreholes. Most of the techniques utilized are also being applied to the gasification of coal veins.

Whether using a direct'air drive or an inverse air injection in situ combustion process it is first necessary to ignite the carbonaceous stratum around a borehole therein before a combustion front can be caused to move thru the stratum. It has been found diflicult to establish a selfsustaining combustion in many carbonaceous strata and in establishing such a combustion zone certain problems have arisen. One method of initiating combustion heretofore utilized comprises heating the Well bore using downhole equipment, such as an electric or gas-fired heater, to raise the temperature of the stratum around the borehole to the ignition point and contacting the hot stratum with air or other O -containing gas so as to ignite the carbonaceous material in the stratum and thereby establish a combustion zone therein. In operating with this method, tar fluidized by the heating frequency flows into the production well and burns therein thereby overheating and damaging the downhole equipment. In other instances the heating of the Walls of the borehole by conventional means has resulted in sealing off the sand face within the borehole by a combination of fluid tar and back pressure built up in the ignition well by expansion of the air and combustion gases therein. In order to establish a self-sustaining combustion zone which can be moved thru a stratum by inverse air injection the initial combustion zone must be established to a sufficient depth to provide a reasonably large heat reservoir to assure sustained combustion with inverse air flow.

The process of the invention is concerned with a method if initiating self-sustaining in situ combustion in a car bonaceous stratum which avoids the above-mentioned difliculties.

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It is accordingly an object of the invention to provide a process for initiating and establishing in situ combustion in a carbonaceous stratum. Another object is to provide a process for initiating combustion in a carbonaceous stratum around a borehole therein which prevents burning of released liquid hydrocarbons in the open borehole. A further object is to avoid sealing the face of an ignition borehole during establishment of in situ combustion in the stratum around the borehole. A further object is to provide a process which establishes a deep and substantial self-sustaining combustion front around an ignition borehole. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the process comprises forming a plurality of fractures in a carbonaceous stratum around a borehole therein, placing within said fractures a mixture of particulate combustible solids and propping material, initiating combustion of said solids and burning same in the fractures so as to heat the adjacent stratum at least to combustion supporting temperature of the carbonaceous material, and injecting air thru the stratum to said borehole to ignite the in-place carbonaceous material and establish in situ combustion therein. The fractures are preferably made in a generally horizontal plane or along the plane of the stratum but they may also be made along generally vertical planes extending radially from the borehole. Fracturing in either manner is conventional in the art for opening up a stratum to greater flow of fluid hydrocarbons into a production borehole. The horizontal fractures in accordance with the invention are positioned at regular intervals in the stratum with from one to two feet of stratum between each pair of fractures. The fractures are filled with a particulate combustible material admixed with a propping agent, such as sand, crushed rock, small ceramic pebbles, etc., conventionally used in propping. Suitable combustion materials comprise particulate coal, charcoal, magnesium, aluminum and any other readily combustible material in the presence of freeoxygen. The use of charcoal in both the fractures and in the well bore packing will be described, but other suitable readily combustible materials may be utilized in lieu thereof.

Another aspect of the invention comprises packing in an ignition borehole a substantial mass of combustible particulate material alone or in admixture with particulate refractory material, or the borehole may be packed with an absorptive bed of solid particles which are absorptive per se or form a bed which absorbs and retains hydrocarbons in liquid form fluidized by the heating process in the Well bore. Here again particulate charcoal, alone, or in admixture with crushed firebrick, coarse sand, crushed rock, porous alumina pebbles, etc., or a bed of these refractory materials without combustible material is packed into the well bore to form an absorptive bed which retains liquid hydrocarbon material driven from the walls of the borehole during the heating thereof.

The combustible solids in the fractures are ignited by any suitable means and air is supplied to the ignited material either by injection thru the ignition borehole or thru boreholes in the stratum closely spaced from the ignition borehole. The combustion of the fuel in the fractures heats the carbonaceous stratum, not only adjacent the borehole wall, but also to a substantial depth of at least several feet from the borehole. As the heating progresses, the ignition temperature of the in-place carbonaceous material is reached and injection of air thru the stratum from surrounding boreholes ignites the carbonaceous material and initiates in situ combustion in the stratum. When the ignition borehole is packed with an absorptive bed of material, heating in the fractures fluidizes and releases inplace carbonaceous material which flows into the absorptive bed of material where it is retained and burned by the injected air, so as to hasten the heating of the stratum and more quickly bring the same to ignition temperature. This prevents burning of the liquid hydrocarbon material in the open borehole and prevents damage to the borehole wall andalso to downhole equipment utilized in the process.

A more complete understanding of the invention may be had by reference to the accompanying schematic drawing of which FIGURE 1 is an elevation in partial section of an arrangement of apparatus and material in an ignition borehole in accordance with one embodiment of the invention; FIGURE 2 is a similar view illustrating another form of apparatus and arrangement for elfecting the invention; and FIGURE 3 is a similar View showing an additional arrangement for effecting the invention.

Referring to FIGURE 1, a carbonaceous stratum such as a tar sand is penetrated by a borehole 12 in which is positioned a casing 14 extending almost to the upper level of stratum v10. A tubing string 16 extends from the .weil head 17 to a lower level in the stratum and is provided with a perforate or slotted section 18, provided with slots 19 on its lower end, substantially co-extensive with stratum 10. A conduit 20 connects with tubing 16 for injection of air or fuel gas as explained hereinafter. A

conduit 21 connects with casing 12 for exhausting gases from the annulus. Borehole 12 is expanded within stratum 10 as at 22, by under-reaming or other method, to provide space for an annular bed of absorptive material 24 around tubing section 18.

Referring to FIGURE 2 a large borehole 12 such as approximately two feet in diameter is drilled thru stratum 1t? and is provided with a casing 14. Regular size casing 26 of about eight inch diameter is positioned axially Within casing 14 and extends to the bottom of stratum 10. The section of casing 26 within stratum 10 is perforated by holes 28 at frequent intervals as shown and an absorptive bed of particulate solids 24 is packed around the perforate section of the casing. Holes 28 are smaller in size than the particulate material in bed 24. Before positioning casing 26 and bed '24 in stratum 10. the stratum is fractured in conventional manner to position fractures 30 at regular intervals thruout the stratum and extending outwardly from the borehole a distance in the range of 10 to 25 feet or more and as each fracture is formed, it is propped and filled with a mixture of particulate combustible solid and propping material. A tubing string 16 extends axially within casing 26 to the upper level of stratum 10 and a conduit 32 connects with casing 26 for injection of air and/ or fuel gas thereto. Conduit 21 connects with casing 12 and the outer annulus.

FIGURE 3 shows an arrangement wherein a borehole 12 is filled with an absorptive bed of particulate material 24 without any axial tubing or casing within the stratum. Here again fractures 30 are filled with a mixture of propping material and particulate combustible solids. Tubing string 16 extends from the well head into the bed of absorptive material or to the upper level thereof. Casing 14 extends from the well head to the top of stratum 10 or thereabouts. Air injection boreholes 36 extend thru the stratum and intersect fractures '30. These air injection boreholes are provided with casing 38 which extends to the upper level of stratum 10 and with tubing 40 extending to a lower level thereof. A heating coil 41, connected to a current source not shown, provides an ignition means.

Operating with the apparatus and arrangement illustrated in FIGURE 1, a bed of charcoal preferably in the form of small briquettes an inch or two in diameter is ignited in any suitable manner as by injecting a mixture of air and gaseous fuel thru tubing 16 and slots 19 into the charcoal mass and igniting the combustible mixture within the mass by a fusee, an electric spark, an electric heating element, or other suitable means (not shown). After ignition of the charcoal, the supply of fuel gas is preferably cut off and the charcoal is burned by injection of air thru tubing section 18 until the whole mass of charcoal is burning by injection of air thru tubing section 18 until the whole mass of charcoal is burning at which time additional air is slowly passed into the charcoal bed from stratum 10 by injection thru one or more surrounding boreholes. As the stratum around borehole 22 is raised substantially in temperature, carbonaceous material is fiuidized therein and passes into the bed of charcoal where it is consumed along with the burning charcoal. During this time, combustion gas is vented thru line 21. As the combustion temperature of stratum :10 is reached, passage of air therethru at an increased rate into borehole 22 ignites the same and establishes in situ combustion therein. After the heating process is well under way and stratum 1t) approaches ignition temperature, the injection of air thru tubing section 18 is preferably terminated and conduit 21 is closed so that tubing 16 can be utilized as an exhaust conduit with combustion products and produced hydrocarbons passing thru slots 19 and into tubing 16 as the combustion front established around borehole 22 is moved outwardly therefrom by the inversely injected air.

If desired, fracturing and burning of fuel in the fractures may be practiced with the arrangement shown in FIGURE 1.

Operation with the arrangement of FIGURE 2 is similar in many respects to that described in FIGURE 1. Ignition of the charcoal or other fuel in bed 24 and in fractures 30 is effected in any suitable manner. One method comprises dropping a substantial mass of burning charcoal on top of bed 24 through the well head by means of a conduit therein (not shown) and injecting air thru lines 21, 32, or 16. .By injecting air thru either conduit 16 or 32 and venting thru conduit 21 the combustion zone migrates from the already ignited charcoal downwardly countercurrently to the flow of air. A preferred. method comprises injecting a combustible premix of fuel gas and air into bed 24 from a ring of surrounding injection boreholes and igniting the mixture as it passes into casing 26 thru perforations 28 by means of a spark, an electric heating element, or a fusee (railroad flare) in conventional manner. The premix burns back thru the charcoal or other solid fuel in bed 24 and in the fractures so as to substantially simultaneously ignite the whole mass of fuel. Thereafter, ignition air is injected thru either conduit 16 or conduit 32 and also thru the surrounding injection boreholes with combustion gas being vented thru line 21. Injection of air thru conduits 16 and 32 serves as a coolant to casing 26 as well as supplying 0 for combustion. As the bed of fuel in the borehole and in the fractures is consumed and the temperature of the adjacent structure reaches the ignition point, injection of air from surrounding boreholes supplies the necessary oxygen for ignition of in-place carbonaceous material thereby starting the in situ combustion process in a deep section of the stratum around borehole 12. During the preheating of the stratum to ignition temperature, hydrocarbon material fluidized in stratum 10 and driven into bed 24 is consumed in the bed along with the solid fuel to aid in the preheating step.

Operation with the arrangement shown in FIGURE 3 comprises igniting the fuel mass in borehole 24 and in fractures 30 as before. One method comprises injecting a combustible premix of propane and air thru tubing 40 in boreholes 36 whereby the combustible mixture passes thru fractures 30 into the bed 24 in borehole 12 and passes either up the annulus or thru tubing 16. By igniting the fuel gas and air mixture by means of igniter 41 as it reaches the upper section of bed 24 the ignited area soon expands thru the bed and thru the fractures so as to initiate combustion in the entire fuel mass. After ignition has been effected, the flow of fuel gas is terminated and the combustion is supported by air injection either thru tubing 16, the annulus surrounding this tubing, or thru tubing 40. In this manner, the combustion of the fuel continues until the temperature of the stratum between borehole 12 and boreholes 36 is raised to ignition temperature at which time the in-place carbonaceous material automatically ignites in the presence of excess air injecte into the stratum.

It is preferable to inject air for the in situ combustion thru tubing 4% thereby establishing combustion in the entire' area between adjacent fractures and between boreholes 12 and boreholes 36. When this has been accomplished, injection of air thru a ring of boreholes more remote from borehole 12 than boreholes 36 is initiated so as to move the combustion zone outwardly toward the injection boreholes. During this phase of the in situ combustion process, produced gases may be vented thru tubing 40, tubing 16, or thru both.

In the arrangement shown in FIGURE 3 injection boreholes 36 are within a few feet of borehole 12 such as 2 to 8 or 10 feet. These boreholes intersect the fractures and there is no danger of blocking of the stratum and shutting off the how of combustion supporting air when utilizing direct injection thru borehole 12; hence, either inverse or direct air injection may be utilized to initiate combustion in the section of stratum comprising the annulus intermediate boreholes 36 and borehole 12. In the arrangement shown in FIGURE 2 or FIGURE 3, bed 24 may be made up entirely of particulate refractory solid material with all of the fuel for the process being provided in fractures 30 supplemented by the hot fluidized tar or other hydrocarbon material driven out of the stratum into the bed of refractory material.

It is to be understood that thermocouples positioned adjacent the wall of the ignition borehole or located at any suitable position within the ignition borehole may be utilized to determine combustion conditions downhole. Pressure sensing means within the production and ignition boreholes are also conventional equipment utilized in this type of process.

I claim:

l. A process for initiating combustion in and producing a permeable carbonaceous stratum which comprises forming a plurality of fractures around an ignition borehole in said stratum extending radially therefrom at least several feet; placing within said fractures to said depth a mixture of particulate combustible solids and propping material; filling said ignition borehole substantially to the top of said stratum with an absorptive bed of particulate solids; injecting combustion-supporting gas into said stratum thru at least one ofiset borehole so as to pass same to said combustible solids; igniting said solids and burning same with said gas so as to heat the adjacent stratum and drive liquid hydrocarbons into said absorptive bed; burning said liquid hydrocarbons in said bed with said injected gas so as to heat the surrounding stratum to combustion supporting temperature whereby sintering of the stratum in the wall of the ignition borehole is avoided; continuing the injection of said gas so as to ignite the surrounding stratum and advance the resulting combustion front generally horizontally thru said stratum toward said injection borehole countercurrently to the flow of gas; and recovering produced hydrocarbons thru said ignition borehole.

2. The process of claim 1 wherein said stratum is fractured horizontally around said borehole at intervals of about 1 to 2 feet.

3. The process of claim 1 wherein air is injected thru a ring of surrounding offset boreholes to sustain the burning of said solids, to ignite the carbonaceous material in said stratum after ignition temperature thereof is reached and to move said combustion front thru said stratum.

4. The process of claim -1 wherein said combustible solid comprises charcoal.

5. The process of claim 4 wherein charcoal is packed into said borehole to form said absorptive bed therein.

6. A process for initiating in situ combustion in a permeable carbonaceous stratum comprising forming a plurality of fractures in said stratum around an ignition borehole therein to a depth of at least several feet; placing within said fractures to said depth a mixture of particulate combustible solids and propping material; packing said borehole with an absorptive bed of particulate solids; burning said combustible solids in said fractures by igniting same and passing air between said ignition borehole and at least one offset borehole spaced within about 2 to 10 feet of said ignition borehole and penetrating said fractures, whereby liquid hydrocarbon flows into said bed and is burned therein and sintering of the stratum adjacent said bed is avoided; continuing the passage of air until said stratum intermediate said boreholes and intermediate said fractures is ignited; and thereafter passing air thru said stratum to the combustion zone from at least one borehole beyond said otfset borehole so as to advance the combustion zone thru said stratum outwardly from the ignition borehole.

7. The process of claim 6 wherein air passing between the boreholes is injected thru a ring of injection boreholes around said ignition borehole including said ofiset borehole; and, after in situ combustion is established, air for advancing the combustion zone is injected thru an outer ring of boreholes.

References ilited in the file of this patent UNITED STATES PATENTS 2,670,047 Mayes et al Feb. 23, 1954 2,818,118 Dixon Dec. 31, 1957 2,734,579 Elkins Feb. 14, 1959 2,901,043 Campion et al Aug. 25, 1-959 

